THE DISCHARGE BEHAVIOR OF SINGLE SYMPATHETIC NEURONS SUPPLYING HUMAN SWEAT GLANDS

Firing properties of single sudomotor axons were studied via tungsten microelectrodes inserted percutaneously into cutaneous fascicles of th e peroneal nerve in awake subjects. Sweating was induced by radiant he at and measured by changes in skin electrical resistance within the in nervation territory on the dorsum of the foot. Eight units were classi fied as sudomotor neurones because spike-triggered averaging revealed a time-locked relationship between the unitary discharge and the subse quent decrease in skin resistance (1.12 +/- 0.05 s), but no relationsh ip to skin blood flow (measured by a laser-doppler probe). Sudomotor u nits usually fired only one (maximum six) spike(s) in a sympathetic bu rst. The mean firing rate was 0.62 Hz, but instantaneous frequencies a bove 50 Hz could be generated. R-wave triggered histograms and coheren ce analysis revealed significant coupling between the firing of three sudomotor neurones and the ECG. Moreover, the firing of four sudomotor neurones showed a weak but significant correlation with the spontaneo us fluctuations in cardiac interval, diastolic pressure, or the rate o f fall in arterial pressure. We conclude that the discharge of human s udomotor neurones is modulated by baroreceptor input.